The present invention relates to a power amplifier for amplifying a high frequency signal of from UHF (Ultra High Frequency) to microwave band, particularly to a high frequency power amplifier preferable by being applied to a transmitting portion of a portable telephone.
According to a portable telephone of mobile communication, there has been remarkable progress in a function and a structure thereof and a reduction in cost and demand therefor has been promoted rapidly. Further, according to a transistor high frequency power amplifier used in a transmitting portion of a portable telephone, under such a situation, small-sized formation, high efficiency formation, and low cost formation have been pursued. Above all, an efficiency (power added efficiency) constituting a ratio of output power to consumption power, constitutes an important characteristic for prolonging a continuous transmission time period of a portable telephone, that is, battery life in speech time since the high frequency power amplifier is a device having the largest consumption power in the portable telephone.
Now, with regard to the high frequency power amplifier, in view of a state of actually using a portable telephone, the high frequency power amplifier is requested to be highly efficient in two different output power levels, described below. One of them is a high output level at a vicinity of a maximum output, specifically, substantially 1 W. This is a level necessary when a portable telephone is remote from a base station or when radio wave is difficult to reach a base station as in a building or the like. Other thereof is a low output level of 10 mW through 100 mW having a large frequency of use in a normal actual used state. There is disclosed a conventional high frequency power amplifier realizing high efficiencies at the two different output levels in, for example, JP-A-336168/1995.
As shown by FIG. 14, the conventional high frequency power amplifier is an amplifier of a two stages constitution using a field effect transistor (hereinafter, referred to as xe2x80x9cFETxe2x80x9d) for a transistor and a post stage thereof is constituted by an output stage 100a of high output using a transistor HA2 and an output stage 100b of low output using a transistor LA3 and either one of them is used by switching. Further, notation IA1 designates an initial stage amplifying transistor and notations IM1, M12, INM3, OM3, and OM2 designate matching circuits. Notations SW1 through SW3 designate switching circuits each constituted by FET, which are inserted in series with high frequency signal paths for carrying out operation of switching high output and low output.
A transistor amplifier is provided with a property in which an efficiency thereof becomes the highest when maximum output determined by a size of the transistor is outputted and therefore, by using transistors having sizes respectively corresponding to high output and low output, high efficiencies can be achieved respectively for high output and low output.
The high frequency power amplifier in FIG. 14 is based on the principle and by making the transistor HA2 larger than the transistor LA3, high power added efficiencies are achieved at respective levels of high output and low output. That is, according to the high frequency power amplifier, at high output time, the transistors IA1 and HA2 are brought into an operational state, the transistor LA3 is brought into a nonoperational state, the switching circuit SW1 is made ON, the switching circuits SW2 and SW3 are made OFF, thereby, the transistor HA2 of a final stage having a larger size outputs high power at high efficiency. Further, at low output time, the transistors IA1 and LA3 are brought into an operational state, the transistor HA2 is brought into a nonoperational state, the switching circuit SW1 is made OFF, the switching circuits SW2 and SW3 are made ON, thereby, the transistor LA3 at a final stage having a smaller size outputs low power at high efficiency.
Further, although the switching circuits SW1 and SW2 do not contribute to high efficiency operation, the switching circuits SW1 and SW2 are used with an object of avoiding parasitic oscillation from being brought about by forming a feedback loop. That is, the switching circuit SW2 is used for cutting a circuit of the output stage 100b in which the transistor LA3 is brought into the nonoperational state from being electrically connected between an output side and an input side of the output stage 100a when the transistor HA2 is brought into the operational state, meanwhile, the switching circuit SW1 is used for cutting a circuit of the output stage 100a in which the transistor HA2 is brought into the nonoperational state from being electrically connected between an output side and an input side of the output stage 100b when the transistor LA3 is brought into the operational state.
According to the above-described conventional technology, when high efficiency formation of the high frequency power amplifier is intended to promote further, power loss caused in the switching circuits SW1 through SW3 inserted in series with the signal lines, cannot be disregarded. In order to reduce the loss in the switching circuit, ON-state series resistance of the switching circuit is obliged to reduce, that is, the size of FET constituting the switching circuit is obliged to enlarge. When three pieces of switching circuits and respective transistors for amplification are mounted to a single piece of a semiconductor chip (semiconductor pellet), which is normally carried out, large-sized formation of such a switching circuit constitutes a hazard in promoting low cost formation by downsizing a semiconductor chips size.
It is an object of the invention to provide a small-sized high frequency power amplifier for preventing oscillation by a small number of switching circuits and outputting high power and low power at high frequencies, a high frequency power amplifier module, and a portable telephone.
A high frequency power amplifier according to the invention for achieving the above-described object is characterized in comprising an amplifying circuit A (first amplifying circuit) and an amplifying circuit B (second amplifying circuit) connected in parallel, in which a size of a transistor at an output stage of the amplifying circuit B is equal to or smaller than xc2xc of that of a transistor at an output stage of the amplifying circuit A and a switching circuit is connected between a signal line forward from the output stage of the amplifying circuit A and the ground terminal. Further, the amplifying circuit A outputs a high frequency signal of high power when the transistor constituting the amplifying circuit B is brought into a nonoperational state and the switching circuit is made OFF and the amplifying circuit B outputs a high frequency signal of low power when the transistor constituting the amplifying circuit A is brought into the nonoperational state and the switching circuit is made ON.
By the above-described constitution, a switching circuit for switching inserted in series with the signal line is dispensed with and the high power and the low power can be outputted with an efficiency higher than that of the conventional example. Such an effect can be achieved by installing the above-described switching circuit and setting the sizes of the transistors at the output stages.
That is, at low output time, by making ON and grounding the switching circuit connected to the signal line on the high output side, a feedback loop is not formed and the parasitic oscillation is avoided. Meanwhile, according to the transistor of the output stage of the amplifying circuit B, when the transistor is brought into the nonoperational state, the parasitic capacitance is small since the size is small. Therefore, at high output time, by high impedance caused by the small parasitic capacitance of the transistor in the nonoperational state, there is not formed a feedback loop to a degree of causing parasitic oscillation and the parasitic oscillation is avoided. In order to avoid the parasitic oscillation, it is preferable that the size of the transistor at the-output stage of the amplifying circuit B is made to be equal to or smaller than xc2xc of that of the amplifying circuit A.
Further, the switching circuit is constituted by a transistor and therefore, the switching circuit can be integrated to the same semiconductor chip along with the transistors for amplification. Further, according to the invention, the switching circuit is for avoiding the parasitic oscillation, a plurality of switching circuits for switching are dispensed with and therefore, an area of the semiconductor chip can be made smaller than that of the conventional example. Therefore, the high frequency power amplifier can be downsized.
Further, with regard to a position of the switching circuit, according to an investigation by the inventors, it is effective for suppressing parasitic oscillation particularly when the switching circuit is disposed between an input terminal of the output stage transistor and the ground terminal or between a matching circuit between an initial stage and the output stage and the ground terminal.